1. Field of the Invention
This invention is directed to polyurethane molding compounds (PUR compounds) which are pore-free or at least substantially pore-free.
2. Description of the Invention
The production of casting resins, for example for the electrical field, by the reaction of organic polyisocyanates with polyhydroxyl compounds is known (cf. for example Kunststoff-Handbuch, Vol. VII, "Polyurethane", by Becker/Braun, Carl Hanser Verlag Munchen, Wien (1983), pages 410 to 425). Moldings produced in this way which are suitable for use as insulators in the electrical industry, have to be basically pore-free and foam-free, so that particular importance is also attributed in the prior art to the careful degassing and drying of the starting materials, particularly the polyhydroxyl compounds. In general, water-absorbing agents such as (anhydrous) zeolites, are added to the reaction mixtures in order safely to prevent bubble formation produced by the reaction of isocyanate groups with water which is accompanied by the evolution of carbon dioxide. On the other hand, the presence of water during hardening is unavoidable in certain applications, for example in the case of cable jointing compounds, which have to be processed in the open, even in poor weather conditions. In this case, pronounced foaming occurs in the case of standard PUR casting compounds based on usual polyetherpolyols with the result that the insulating effect of the casting resin is seriously impaired, even when measures are taken to ensure that the starting materials are free from water by storing the starting materials in the absence of atmospheric moisture and adding water-absorbing agents.
Foaming does not occur when castor oil is used instead of polyether polyols. However, this natural product is subject to considerable quality fluctuations and, in addition, PUR molding compounds based on this polyester polyol are extremely vulnerable to hydrolysis whenever the hardened molding material comes into contact with moisture. Another disadvantage of compounds based on castor oil is that they turn brittle during prolonged storage and relatively high temperatures (above 50.degree. C.). The casting resin compounds gradually develop cracks which adversely affect their electrical insulating power.
Glass-fiber-containing laminates of polyisocyanates and commercially available polyether polyols are porous and hence unusable. Bubble formation cannot be prevented even by careful drying of the glass fibers and polyether polyols or by the addition of water-absorbing agents.
Fatty acid-modified polyisocyanates and fatty acid-modified polyurethanes are also described as paint binders in the literature (Kunststoff Handbuch, No. 7, Polyurethane, 1983, pages 86, 87). These binders are solid resins or resins of very high viscosity which are dissolved in paint solvents and applied in this form. Due to their solvent content, they are unsuitable for the production of polyurethane resin molding materials in accordance with the present invention using low viscosity, solvent-free components.
Accordingly, an object of the present invention is to provide an improved process for the production of pore-free polyurethane molding compounds which, in particular, are also suitable for processing together with fibers, particularly glass fibers.